The present invention relates to a continuous process for producing an alkylene oxide by direct oxidation of an alkene with oxygen in the presence of a silver-comprising catalyst, wherein the process comprises an integrated treatment of the catalyst for increasing its activity and/or selectivity. The invention likewise relates to the use of such a process for regenerating a spent catalyst previously used for alkylene oxide production.
Alkylene oxides are important key chemicals which have varied uses. Various production processes for producing alkylene oxides are known in principle in the prior art. Alkylene oxides can be produced, for example, by reacting olefins with hydroperoxides, e.g. with hydrogen peroxide, or by direct oxidation with oxygen.
Ethylene oxide is produced industrially frequently by direct oxidation of ethylene with oxygen in the presence of silver-comprising catalysts. Frequently, supported catalysts are used onto which the catalytically active metallic silver was applied by means of a suitable process. As support material, use can be made in principle of various porous materials such as, e.g., activated carbon, dioxides of titanium, zirconium or silicon, or ceramic compositions or mixtures of these materials. Generally, α-aluminum oxide is used as support material.
In addition to silver and support material, these catalysts mostly comprise promoters for improving the catalytic properties. According to the prior art, for example alkali metal and/or alkaline earth metal compounds are applied to the support in small amounts. Some publications teach the use of transition metals such as tungsten or molybdenum. A particularly preferred promoter in the case of silver catalysts is rhenium. Catalysts which comprise rhenium and/or other transition metal promoters in combination with alkali metal and/or alkaline earth metal compounds are preferentially used in industry because of their high selectivity.
In the course of time a number of processes have been developed for the direct oxidation of alkenes, in particular ethylene, using various silver catalysts with the purpose of beneficially affecting the selectivity and/or activity. Selectivity is taken to mean the molar percentage of alkylene which reacts to form alkylene oxide. The activity is characterized by the alkylene oxide concentration at the reactor outlet under otherwise constant conditions, such as, for example, temperature, pressure, gas rate, catalyst rate. The higher the alkylene oxide concentration, the higher is the activity. The lower the temperature which is required to reach a certain alkylene oxide concentration, the higher is the activity. Owing to the large amounts of, for example, ethylene oxide which are produced in industrial processes by the direct oxidation process, any increase in selectivity or activity of a catalyst is of considerable economic importance. In addition to the activity, the service life of the catalyst is of enormous economic importance. The activity and/or selectivity of known catalysts decrease with advancing time of use, so that finally an economically unfavorable change of catalyst must proceed.
Processes for improving the activity and/or selectivity of spent silver catalysts are mostly based on post-treating or regenerating a spent catalyst. The post treatment comprises, for example, impregnating the spent catalyst with a solution comprising water, a water-miscible solvent and a cesium and/or rubidium compound. The catalyst which is impregnated in this manner is dried and the catalyst thus dried is then used again in the oxidation reaction. Such processes are described, for example, in DE 25 19 599, DE 26 11 856, DE 26 36 680, DE 26 49 359, EP 0 101 008 and DE 29 38 245. In the regeneration processes which are described in U.S. Pat. No. 4,529,714 or U.S. Pat. No. 4,391,735, the silver-comprising catalyst is treated in a very similar manner; but the regeneration solution here comprises additionally hydrazine and/or aliphatic or aromatic acids as additives.
However, the processes described have the disadvantage that the oxidation process must be interrupted for the entire period of treatment of the catalyst. A process in which activity and/or selectivity of the silver catalyst can be improved in situ by a suitable treatment at a given time during the running process appears to be advantageous. The expression “in situ”, as used in the context of the invention, means a treatment of the catalyst is carried out during the running oxidation reaction without the oxidation process being interrupted.
An in situ regeneration process is described in U.S. Pat. No. 6,624,116. This document relates to a process for regenerating a catalyst which is used in an autothermal oxidation process for converting paraffin hydrocarbons to olefins. The regeneration is achieved by introducing a vaporizable compound of the metals of group 8b and/or by introducing a vaporizable promoter into the reactor together with the reaction mixture. Silver-comprising catalysts and processes for the direct oxidation of alkenes to alkylene oxides are not described in U.S. Pat. No. 6,624,116.
There was therefore the need for advantageous processes for producing alkylene oxides by direct oxidation of alkenes in the presence of silver-comprising catalysts in which the above described disadvantages, that is, for example, frequent production outage and/or losses of activity and/or selectivity during advancing period of use, are minimized. In addition, there was the need for an advantageous regeneration process for silver-comprising catalysts which are used in the direct oxidation of alkenes.
Surprisingly, it has now been found that the selectivity and activity are beneficially affected by an in-situ treatment of a silver-comprising catalyst with an ethanol-comprising mixture which is carried out during the production of an alkylene oxide by direct oxidation of an alkene with oxygen.